There is a need for a low temperature refrigerant for use in cold stores. Prior to the Montreal Protocol, this function was filled by R502, an azeotrope of R115 and R22. This refrigerant was particularly attractive in low temperature situations where R12 (CC12F2) or R22 were reaching their effective working limits. At these low temperatures it was possible to achieve a significant increase in capacity over that obtainable with R22 with a major benefit being operation at considerably lower discharge temperatures. However, since R502 contains R115, which is a strong ozone depleter, it is now no longer available for use.
Subsequently, this requirement has been partially met by using two blends containing R143a. The first is R404A, which consists of R125 (44% w/w), R143a (52% w/w) and R134a (4% w/w). The second is R507A, which consists of an azeotropic mixture of R125 (50% w/w) and R143a (50% w/w).
The problem with these blends is that they have very high global warming potentials (GWP).
The concept of a Global Warming Potential (GWP) has been developed to compare the ability of a greenhouse gas to trap heat in the atmosphere relative to another gas. Carbon dioxide (CO2) has been chosen as the reference gas. Since GWPs are ratios, they are dimensionless. The GWPs quoted below are those given in IPCC-1995 for 100 year time horizons. The GWPs for blends are calculated by summing the products of the mass fraction times the individual component's GWP.
A greenhouse gas is a gas that causes the Earth's atmosphere to trap heat. The greenhouse gas allows the sun's radiation to reach the Earth's surface. The Earth's surface is heated by this radiation and emits longer wavelength infra-red radiation due to the heating. The greenhouse gas now prevents this radiation from escaping back into space by absorbing it and hence traps it in the atmosphere.
R507 has a GWP of 3300 and R404A is only slightly less at 3260. These high GWPs are due to the presence of R143a. Pure R143a has a GWP of 3800 compared to that of the other main component, R125, which is only 2800.
R22 alone has also been used, but this is an ozone depleter that will be phased out over the next decade. Also, the efficiency of R22 at the low temperatures required for cold storage is poor.
There is now considerable concern about global warming and, hence, it is important to use blends with as low a GWP as possible. Clearly there is a need to find a substitute for R502, which is not an ozone depleter, has a low GWP and can operate more efficiently at the low temperatures required than R22, R404A or R507.